This patch removes various technical debt in the form of "???" comments
throughout the GNAT sources.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* exp_ch6.adb (Add_Simple_Call_By_Copy_Code): Add comments
	regarding special handling of components which depend on
	discriminants.
	* exp_dist.adb (Build_From_Any_Function): Add Real_Rep actual
	for calls to Has_Stream_Attribute_Definition.
	(Build_To_Any_Function): Likewise.
	(Build_TypeCode_Function): Likewise.
	* freeze.adb (Freeze_Entity): Add missing comment for Test_E.
	* libgnat/s-utf_32.adb: Remove disabled warning comments and
	temporarily inserted pragma warnings.  Remove very old (2006 and
	2012) comments about bootstrapping older versions.
	* par.adb (P_Identifier): Add new parameter Force_Msg.
	* par-ch2.adb (P_Identifier): Restructure and clean up function.
	* par-ch3.adb (P_Defining_Identifier): Remove code duplication
	for parsing identifiers.
	* sem_attr.adb (Stream_Attribute_Available): Add missing
	comments and add Real_Rep actual for calls to
	Has_Stream_Attribute_Definition.
	* sem_cat.adb (Has_Read_Write_Attribute): Add Real_Rep actual
	for calls to Has_Stream_Attribute_Definition.
	(Has_Stream_Attribute_Definition): Remove local Real_Rep and fix
	recursive calls. Add default value for Real_Rep.
	* sem_cat.ads (Has_Stream_Attribute_Definition): Add new out
	parameter "Real_Rep".
	* sem_type.adb (Add_Entry): Add condition to avoid passing
	non-function calls to Function_Interp_Has_Abstract_Op.
	(Function_Interp_Has_Abstract_Op): Add missing comments and
	remove check for Is_Overloadable.
	* sem_util.adb (Derivation_Too_Early_To_Inherit): Remove
	duplicated code.